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Posted by PerlFAQ Server on May 23, 2008, 3:03 am
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This is an excerpt from the latest version perlfaq5.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .
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5.15: How come when I open a file read-write it wipes it out?
Because you're using something like this, which truncates the file and
*then* gives you read-write access:
open(FH, "+> /path/name"); # WRONG (almost always)
Whoops. You should instead use this, which will fail if the file doesn't
exist.
open(FH, "+< /path/name"); # open for update
Using ">" always clobbers or creates. Using "<" never does either. The
"+" doesn't change this.
Here are examples of many kinds of file opens. Those using sysopen() all
assume
use Fcntl;
To open file for reading:
open(FH, "< $path") || die $!;
sysopen(FH, $path, O_RDONLY) || die $!;
To open file for writing, create new file if needed or else truncate old
file:
open(FH, "> $path") || die $!;
sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!;
sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!;
To open file for writing, create new file, file must not exist:
sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
To open file for appending, create if necessary:
open(FH, ">> $path") || die $!;
sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!;
sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;
To open file for appending, file must exist:
sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
To open file for update, file must exist:
open(FH, "+< $path") || die $!;
sysopen(FH, $path, O_RDWR) || die $!;
To open file for update, create file if necessary:
sysopen(FH, $path, O_RDWR|O_CREAT) || die $!;
sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!;
To open file for update, file must not exist:
sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!;
sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!;
To open a file without blocking, creating if necessary:
sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT)
or die "can't open /foo/somefile: $!":
Be warned that neither creation nor deletion of files is guaranteed to
be an atomic operation over NFS. That is, two processes might both
successfully create or unlink the same file! Therefore O_EXCL isn't as
exclusive as you might wish.
See also the new perlopentut if you have it (new for 5.6).
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The perlfaq-workers, a group of volunteers, maintain the perlfaq. They
are not necessarily experts in every domain where Perl might show up,
so please include as much information as possible and relevant in any
corrections. The perlfaq-workers also don't have access to every
operating system or platform, so please include relevant details for
corrections to examples that do not work on particular platforms.
Working code is greatly appreciated.
If you'd like to help maintain the perlfaq, see the details in
perlfaq.pod.
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| Similar Threads | Posted | | FAQ 5.13: How come when I open a file read-write it wipes it out? | December 24, 2004, 6:03 am |
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